Reaching Higher: Timber Towers Are Changing the Construction Landscape

The interest and use of wood as a building material has been around for centuries. Architects, designers, builders and window manufacturers, among others, have often touted the “warmth of wood” in their designs and other promotional materials. Aesthetics aside, several other reasons drive growth. These include sustainability, energy efficiency and durability. Timber is also considered a “carbon sink,” meaning it absorbs more
carbon from the atmosphere than it releases, making it an attractive option given increasing energy performance requirements.

Until a few years ago, however, there were height limits on timber structures. But the 2021 International Building Code gave the greenlight to tall wood buildings up to 18 stories. Since then, timber buildings and timber-hybrid buildings have been on the rise. According to the Council on Tall Buildings and Urban Habitats (CTBUH), the construction industry
is seeing significant momentum world-wide toward mass-timber buildings. CTBUH research on the state of tall timber, as of 2022, shows 139 mass timber buildings worldwide, eight stories or higher, either complete, under construction or proposed. Europe is the current leader in timber/timber-hybrid buildings, with 71% of these structures.

Timber Takeover

Until recently, Mjøstårnet, an 18-story mixed-use building in Brumunddal, Norway, was the world’s tallest timber building, standing 280 feet tall. Norwegian studio Voll Arkitekter was the architect of the building, which was completed in 2019. Also in Europe and completed in 2020, the 276-foot HoHo building in Vienna, Austria, designed by RLP Rüdiger Lainer + Partner, was the previous tallest concrete-timber hybrid building. It took just four feet for the 284-foot Ascent in Milwaukee to upset the charts. In July 2022, the CTBUH declared the 25-story tower as the world’s tallest timber-concrete hybrid building and the tallest timber building overall.

Designed by Korb + Associates Architects (KAA) in Milwaukee, Ascent broke ground in August 2020, followed by about two years of research, testing, planning and collaboration with those involved. C.D. Smith in Fond du Lac, Wis., was the general contractor, and New York-based Thornton Tomasetti was the structural engineer.

Glass Interaction

According to Jason Korb, owner and principal architect, for the entire lifeline of the firm’s involvement, the project was planned to be a tall timber building. Glass also played an essential role along with timber. Korb says using glass was important because the developer, New Land Enterprises, designs buildings from the inside out.

“One of the major attractions/amenities they wanted to offer with this building was a great floor-to-ceiling line in every occupied space,” says Korb. “The amount of clear glass has served to showcase the wood superstructure, especially in the evening.”

The project features 84,000 square feet of SunGuard SuperNeutral SN68 coating on clear glass from Guardian Glass. Oldcastle BuildingEnvelope® (OBE) fabricated the insulating glass units and supplied the curtainwall and window systems. Klein-Dickert, based in Pewaukee, Wis., was the contract glazier.

Derek Wiencek with Klein-Dickert, said working on the project wasn’t all that different compared to a more traditional structure. Instead of anchoring to concrete, he says they anchored to the cross-laminate timber (CLT). He adds that the first few floors are concrete, and the higher floors are CLT, so there was a slightly larger transition to the first CLT floor.

Kevin Rogers, KAA senior project manager, adds that when working on timber projects the speed of installation of the window wall system is critical to enclose/protect the exposed timber from the elements.

Korb and Rogers also say that special considerations were necessary to keep the dew point away from the wood superstructure, resulting in a 6-mullion dye redesign. “In spandrel conditions, at floor edges, the dew point is critical,” they say.

In addition to aesthetics, timber buildings meet an important environmental detail. Liz Syrrakos, operations manager with KAA, says the use of mass timber is on the rise due to its carbon-neutral impact.

“Mass timber as a structural system as opposed to a standard steel or concrete makes this project a model for sustainable development,” she says. “Mass timber is a carbon sink, and Ascent’s use of mass timber is estimated as having the same impact as taking 2,350 cars off the road for one year. The significance of this impact, combined with the desire to incorporate the element or practice of biophilia in our design, highly drove the desire to design with mass timber.”

Wiencek agrees that the sustainability factor is a standout feature of the Ascent and that other timber buildings are already under construction in Milwaukee.

He adds that one key takeaway is the look from the inside and the beauty of all the wooden beams and interiors.

Ascent also received the Innovation Award from WoodWorks Wood Product Council and LEED Silver certification.

Special Considerations: Glazing and Timber

Mass timber currently represents only a small percentage of new commercial buildings in the U.S., but it could be the next big thing in construction. According to Allied Market Research, the global mass timber construction market was valued at $857.1 million in 2021, and is projected to reach $1.5 billion by 2031, growing at a compound annual growth rate of 6% from 2022 to 2031.

Timber’s aesthetics combined with eco-friendly benefits have made it an attractive alternative to traditional materials. Timber structures, however, also bring a number of unique considerations, including some involving the glazing.

David Barber, a principal with Arup, is the co-author of a Façade Tectonics paper titled “Fire Safety and Code Challenges for Mass Timber in Curtainwall Systems.” He says timber projects using window walls are normally constructed and detailed in a relatively similar way to steel and concrete construction. He adds that top of the wall connections can be slightly different if connecting into the underside of the mass timber.

With curtainwall, however, he says there are quite a number of issues and differences.

“The gap at the floor to the back of the curtainwall requires a fire seal and
there are very few products that have been fire tested,” he says. “… The fire
sealing at this gap is not very clearly explained in the International Buildling
Code for timber, there are few architectural details published as guidance
and the very limited fire stopping solutions means there is a lot of variation.”

He continues, “This also results in issues with building approvals, with some curtainwalls being ruled out, others having an over-designed fire seal and others having a fairly straight forward solution. There are also other code-related issues as concealed spaces for mass timber buildings need to be addressed and that can be interpreted differently.”

What is Cross-Laminated Timber?

According to Think Wood, which provides resources to architects, developers, and contractors, cross-laminated timber (CLT) panels consist of layered lumber boards stacked crosswise at 90-degree angles and glued into place. The board thickness typically varies between 5/8 to 2 inches, with widths from 2.4 to 9.5 inches.

Ellen Rogers is the editorial director of USGlass magazine.
Email her at erogers@glass.com and connect with her on LinkedIn.

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